Category: Human Perception

Sensory synesthesia is a neurological phenomenon in which stimuli from one modality of sensory input leads to involuntary and automatic experiences in another sensory modality. There is some debate regarding the classification of synesthetic phenomenon, but several striking observations reveal that at least a small percentage of people experience a heightened interconnectedness between their different senses.

Kiki, Bouba, and Visual Perception

In 1929, the German-American scientist Wolfgang Kohler observed what is now known as the Bouba-Kiki effect [1]. In 2001, Vilyanur S. Ramachandran replicated Kohler’s experiment with college students in the United States and India, and found a large consensus between participants prompted to provide auditory names to visual objects [2]. The findings of Ramachandran and Kohler demonstrate that sensory information appears to carry a predictable and consistent scaffolding of associations and relationships to other modalities of stimuli. The participants’ visual perceptions of the shapes printed on the page were used to make judgments of the appropriate auditory sounds that ought to be associated with those shapes. VS Ramachandran and his colleague Edward Hubbard suggest that the evolution of language may not be entirely arbitrary—instead, the naming of objects in space may reflect a natural association of auditory stimuli with the visual, tactile, olfactory, and overall perception of the object’s nature. Sounds (and by extension, all sensory information) may automatically convey some degree of symbolic meaning in relation to experiences from other senses.

Auditory-Tactile Synesthesia

When viewed with an MRI, the brain activity of a patient with a localized lesion in the right ventrolateral nucleus of the thalamus revealed modifications to the individuals’ perception. “Initially, the patient was more likely to detect events on the contralesional side when a simultaneous ipsilesional event was presented within the same, but not different sensory modality.” Eventually, this transformed into a form of synesthesia “in which auditory stimuli produce tactile percepts.” This study revealed the likelihood that the experience of sensory synesthesia may be acquired after a brain injury [3].

Visual-Tactile Synesthesia

Mirror-touch synesthesia is a condition in which watching another person being touched activates a similar neural circuit to actual touch. When observing individuals who experience mirror-touch synesthesia with brain imaging, their empathic responses to the experiences of other people appears to be heightened [4]. This form of synesthesia also appears to augment an individual’s ability to recognize and interpret the facial expressions of an interaction partner [5]. Although a thorough empirical explanation for the phenomenon has not yet been developed, there are different potential theoretical explanations currently being investigated in more detail. The Threshold Theory explains it “in terms of hyper-activity within a mirror system for touch and/or pain,” and the Self-Other Theory explains it “in terms of disturbances in the ability to distinguish the self from others.” [6] The two theories carry different implications: the Threshold Theory implies a localized phenomenon impacting the mirror system, while the Self-Other theory implies a more general difference that may be reflected in other cognitive processes as well.

Enhanced Sensory Perception

Some scholars argue that artistic experimentation may be rooted in sensory synesthesia, by allowing an artist to describe a sensory experience using a wider range of detail [7]. Although scientists have developed methods of testing and profiling synesthetes [8], much of the theoretical framework used to understand cross-modal sensory perception remains speculative. Although VS Ramachandran mentions a possible relationship between synesthesia and enhanced sensory perception [9], it remains unclear exactly how this enhancement manifests itself in a person’s ability to perform different activities or pursue artistic endeavors. In a preliminary study exploring the perceptual processing abilities of synaesthetes [10], “there was a relationship between the modality of synaesthetic experience and the modality of sensory enhancement.” In other words, a synaesthete who experiences color triggered by other sensory modalities will also have enhanced color perception. A synaesthete who experiences tactile sensations will have enhanced tactile perception. Further research is required to understand exactly how these enhanced perceptual abilities manifest themselves in common tasks.

People rapidly and accurately identify 3-dimensional objects using only their sense of touch [1]. This process occurs through a sequence of exploratory procedures that underlie the cognitive strategies used to conclusively identify an object entirely through its haptic features. The following four procedures are the most common methods for tactile exploration [2]. Continue reading “How Humans Explore Objects via Touch”

If we’re honest about shortcomings in human physiology, optical illusions would be labelled “Brain Failures.”

– Neil DeGrasse Tyson

When we think about the ways our perception plays tricks on us, optical illusions come to mind first. They’re not the only kind of sensory illusions, though. Tactile illusions also illustrate the fascinating ways that our perception ‘fails’ to reflect reality. In this post, we describe several different types of tactile illusions.

When we think about the content of a conversation, it’s easy to focus on just the verbal information exchanged through spoken words; however, there are many other factors that color our interpretation of conversations and, in turn, the information they communicate. One such consideration is the context provided by prosody—the intonation, stress, tempo, rhythm, and pauses in a person’s speech, all of which lend their voice a unique texture. The brain also employs detailed mappings that link different kinds of facial expressions and gestures with the emotions and nuances that they convey. In fact, up to 65% of the raw information in a conversation is exchanged nonverbally [1]. As we continue to investigate human communication, we uncover a highly complex, multi-modal system that comprises many of our senses—including our sense of touch.

Expressive aphasia, also known as Broca’s aphasia, is a neurological condition characterized by an individual’s inability to produce grammatically correct speech, often due to a physical impact or alteration to the anterior regions of the brain, which impairs the proper function of neurons that would otherwise help construct vocalizations of grammatically correct sentences [1]. In the suburbs of Paris, France in 1861, Paul Broca identified the location of the region responsible for expressive aphasia after he conducted an autopsy of a patient incapable of uttering any word other than “tan” [2]. Though speculations of the structure and function of human consciousness had existed for several centuries, Broca’s discovery resulted in a new framework for understanding the brain’s role in producing conscious experience. The patient’s brain incurred a lesion from injury, and only a small subset of his cognitive function was impaired. Naturally, psychologists concluded that different parts of the brain mediate different cognitive processes. By 1874, Carl Wernicke discovered receptive aphasia (Wernicke’s aphasia), which results from damage to posterior regions of the brain [3], and increasing numbers of scientists began exploring which regions of the brain were responsible for different aspects of cognition. Brain science adopted a new goal: mapping the locations of the brain corresponding to each observable function in human consciousness.

Though haptic feedback often relies on the mapping of information directly to a stimulus produced by tactile actuators, we can often leverage human sensory illusions to produce a richer tactile experience for users. One such illusion is the “Cutaneous Rabbit Illusion” (also known as sensory saltation):

The cutaneous rabbit illusion (also known as cutaneous saltation and sometimes the cutaneous rabbit effect or CRE) is a tactile illusion evoked by tapping two or more separate regions of the skin in rapid succession. The illusion is most readily evoked on regions of the body surface that have relatively poor spatial acuity, such as the forearm. A rapid sequence of taps delivered first near the wrist and then near the elbow creates the sensation of sequential taps hopping up the arm from the wrist towards the elbow, although no physical stimulus was applied between the two actual stimulus locations.

This illusion allows us to simulate the experience of movement across the skin using a smaller number of vibration motors – if we want to communicate direction or spatial relationships, sensory saltation allows us to quickly deliver the information by letting the human brain fill in the gaps.